qemu: query QEMU for migration blockers before our own harcoded checks

qemu: query QEMU for migration blockers before our own harcoded checks

Since QEMU 6.0, if QEMU knows that a migration would fail,
'query-migrate' will return an array of error strings describing the
migration blockers.  This can be used to check whether there are any
devices/conditions blocking migration.

This patch adds a call to this query at the top of
qemuMigrationSrcIsAllowed().

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>